Construction of Ag3PO4/g-C3N4 Z-Scheme Heterojunction Composites with Visible Light Response for Enhanced Photocatalytic Degradation

Ag3PO4/g-C3N4 photocatalytic composites were synthesized via calcination and hydrothermal synthesis for the degradation of rhodamine B (RhB) in wastewater, and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance spectroscopy (DRS). The degradation of RhB by Ag3PO4/g-C3N4 composites was investigated to evaluate their photocatalytic performance and cyclic degradation stability. The experimental results showed that the composites demonstrated notable photocatalytic activity and stability during degradation. Their high degradation efficiency is attributed to the Z-scheme transfer mechanism, in which the electrons in the Ag3PO4 conduction band and the holes in the g-C3N4 valence band are annihilated by heterojunction recombination, which greatly limits the recombination of photogenerated electrons and holes in the catalyst and enhances the activity of the composite photocatalyst. In addition, measurements of photocurrent (PC) and electrochemical impedance spectroscopy (EIS) confirmed that the efficient charge separation of photo-generated charges stemmed from strong interactions at the close contact interface. Finally, the mechanism for catalytic enhancement in the composite photocatalysts was proposed based on hole and radical trapping experiments, electron paramagnetic resonance (EPR) analysis, and work function evaluation.


Introduction
The utilization of nanomaterials in semiconductor photocatalytic degradation of organic pollutants offers high efficiency and low cost, making it a promising approach [1][2][3][4].Recently, silver phosphate (Ag 3 PO 4 ) has garnered significant attention for its outstanding photocatalytic performance in degrading organic pollutants under visible light irradiation [5].Nevertheless, Ag 3 PO 4 typically faces challenges such as high carrier complexity, severe photocorrosion, and a relatively large particle size, which notably restrict its photocatalytic performance [6][7][8].In order to overcome these shortcomings and improve its photocatalytic efficiency and durability, the formation of a semiconductor heterojunction has been employed to enhance both the photocatalytic performance and stability of Ag 3 PO 4 .
It has been demonstrated that the heterojunction structure based on Ag 3 PO 4 can effectively enhance the separation of photogenerated electron hole pairs, thereby promoting the generation of more reactive oxygen species [9,10].This enhances the applicability of Ag 3 PO 4 in solar photocatalysis.Therefore, constructing a Z-scheme is an effective method to enhance the photocatalytic activity of Ag 3 PO 4 [11][12][13].
Graphitic carbon nitride (g-C 3 N 4 ), a promising visible light-responsive photocatalyst, has been widely used in photoelectrocatalysis because of its stable chemical properties and remarkable responsiveness to visible light [14,15].However, g-C 3 N 4 is plagued by a wide band gap, high carrier complexity, and limited visible light response range [16,17].The modification of g-C 3 N 4 -based photocatalysts can be investigated by expanding their light absorption range, which requires a narrower band gap.Furthermore, enhancing the acquisition or retention of electrons and holes with potent redox capability requires the presence of a more positive valence band and a more negative conduction band potential.Therefore, g-C 3 N 4 forms heterojunctions with other semiconductors, but acquiring or retaining electrons and holes with potent redox capability demands a more positive valence band and a more negative conduction band potential.This not only broadens the light absorption range but also modulates the energy band structure to enhance the efficiency of charge-carrier separation.This results in a substantial reduction in the carrier complexation rate and an increase in the collection and utilization of visible light, significantly boosting the photocatalytic activity [18,19].
Constructing heterostructures for photocatalysts has emerged as a mainstream approach to enhance visible light utilization and expedite charge transfer and separation, thus overcoming the drawbacks of g-C 3 N 4 .Fewer factors are considered for constructing g-C 3 N 4 heterojunctions [20] and the selected semiconductor generally only needs to have a suitably matched energy band structure.Based on the above analysis, the forbidden band widths of Ag 3 PO 4 and g-C 3 N 4 result in a staggered alignment, enabling the formation of an Ag 3 PO 4 /g-C 3 N 4 heterojunction that demonstrates excellent photocatalytic activity under visible light irradiation [21][22][23].
Therefore, according to the previous research on Ag 3 PO 4 /g-C 3 N 4 composites, the band structure of the semiconductor was determined by using the small angle valence band and tauc plots in combination with the Mott-Schottky curves.The direction of photogenerated carrier transfer was inferred from the work function, and further analysis focused on the catalytic degradation mechanism of the composites.
In this study, Ag 3 PO 4 /g-C 3 N 4 heterojunction composites were prepared via simple calcination and hydrothermal methods.Simulated RhB solution was used to assess the degradation of organic pollutants and their role in RhB photodegradation.The experimental results confirmed that Ag 3 PO 4 /g-C 3 N 4 composites exhibited high photocatalytic activity.Additionally, various advanced characterization techniques were used to characterize and analyze the Ag 3 PO 4 /g-C 3 N 4 composites.Subsequently, a potential light-driven catalytic enhancement mechanism for Ag 3 PO 4 /g-C 3 N 4 was proposed.

Structure and Microscopic Morphology of the Samples
As can be seen from Figure 1a, two diffraction peaks of g-C 3 N 4 appear at 2θ of 13.2 • and 27.6 • , corresponding to crystal planes (100) and (002), respectively [24,25].The peak at 13.1 • corresponds to the planar stacking structure of tri-s-triazine units within g-C 3 N 4 , while the diffraction peak at 27.6 • is related to the interlayer stacking of graphitelike structures.These observations are consistent with previous research, confirming the successful preparation of graphitic phase carbon nitride.The 2θ of the diffraction peaks in the Ag 3 PO 4 sample were 20.9   , 69.9 • , and 71.9 • , which corresponded to the diffraction peaks in the Ag 3 PO 4 standardized atlas (PDF# 06-0505) of (110), (200), (210), (211), (310), ( 222), (320), (321), (400), (420), and (421) crystal planes.The absence of additional peaks confirms the high purity of the prepared Ag 3 PO 4 [26,27].The phase of Ag 3 PO 4 remained unchanged in the composite catalyst Ag 3 PO 4 /gC 3 N 4 , whereas the (100) crystalline surface of g-C 3 N 4 disappeared.This may be due to the low diffraction intensity of the (100) crystalline surface or the insufficient range of detection values [28].It is noteworthy that the intensity of the Ag 3 PO 4 diffraction peaks appeared reduced in Ag 3 PO 4 /g-C 3 N 4 , indicating that the introduction of g-C 3 N 4 did not alter the material's structure.The EDS energy spectrum in Figure 1b confirms the presence of Ag, P, C, N, and O elements in the sample.In order to obtain the specific surface area of the prepared adsorbent materials, BET specific surface area tests were carried out on g-C3N4 and Ag3PO4/g-C3N4, and the results are shown in Figure 1c.Both g-C3N4 and Ag3PO4/g-C3N4 exhibited H3-type hysteresis loops in their N2 adsorption and desorption isotherms, characteristic of type IV isotherms.These observations were attributed to the stacking of Ag3PO4 particles and massive g-C3N4.This phenomenon can be attributed to the formation of slit holes in the Ag3PO4 particles and massive g-C3N4.The composite of g-C3N4 and Ag3PO4 exhibited a significantly increased specific surface area.An increased specific surface area enhances the availability of charge transfer channels and catalytic active sites.
The morphology of Ag3PO4 particles, illustrated in Figure 2a, appears irregularly spherical and agglomerated.As shown in Figure 2b, the pure g-C3N4 is an aggregated flocculent structure, which has the typical characteristics of g-C3N4 synthesized by thermal polymerization.When Ag3PO4 and g-C3N4 were combined (Figure 2c), their basic morphological features remain unchanged, resulting in a compact composite structure.In addition, Figure 2d reveals the close contact between Ag3PO4 and g-C3N4, forming a heterojunction with a crystal plane spacing of 0.2689 nm, which is consistent with the (210) crystal plane spacing of Ag3PO4.Notably, the high specific surface area of g-C3N4 facilitates enhanced adsorption of pollutant molecules during photocatalytic reactions.Simultaneously, the heterostructure formed between g-C3N4 and Ag3PO4 facilitates the separation of photogenerated electrons and holes, thereby enhancing the photocatalytic activity [29,30].In summary, the successful construction of the photocatalytic composite Ag3PO4/g-C3N4 heterojunction is demonstrated.In order to obtain the specific surface area of the prepared adsorbent materials, BET specific surface area tests were carried out on g-C 3 N 4 and Ag 3 PO 4 /g-C 3 N 4 , and the results are shown in Figure 1c.Both g-C 3 N 4 and Ag 3 PO 4 /g-C 3 N 4 exhibited H 3 -type hysteresis loops in their N 2 adsorption and desorption isotherms, characteristic of type IV isotherms.These observations were attributed to the stacking of Ag 3 PO 4 particles and massive g-C 3 N 4 .This phenomenon can be attributed to the formation of slit holes in the Ag 3 PO 4 particles and massive g-C 3 N 4 .The composite of g-C 3 N 4 and Ag 3 PO 4 exhibited a significantly increased specific surface area.An increased specific surface area enhances the availability of charge transfer channels and catalytic active sites.
The morphology of Ag 3 PO 4 particles, illustrated in Figure 2a, appears irregularly spherical and agglomerated.As shown in Figure 2b, the pure g-C 3 N 4 is an aggregated flocculent structure, which has the typical characteristics of g-C 3 N 4 synthesized by thermal polymerization.When Ag 3 PO 4 and g-C 3 N 4 were combined (Figure 2c), their basic morphological features remain unchanged, resulting in a compact composite structure.In addition, Figure 2d reveals the close contact between Ag 3 PO 4 and g-C 3 N 4 , forming a heterojunction with a crystal plane spacing of 0.2689 nm, which is consistent with the (210) crystal plane spacing of Ag 3 PO 4 .Notably, the high specific surface area of g-C 3 N 4 facilitates enhanced adsorption of pollutant molecules during photocatalytic reactions.Simultaneously, the heterostructure formed between g-C 3 N 4 and Ag 3 PO 4 facilitates the separation of photogenerated electrons and holes, thereby enhancing the photocatalytic activity [29,30].In summary, the successful construction of the photocatalytic composite Ag 3 PO 4 /g-C 3 N 4 heterojunction is demonstrated.In order to obtain the specific surface area of the prepared adsorbent materials, BET specific surface area tests were carried out on g-C3N4 and Ag3PO4/g-C3N4, and the results are shown in Figure 1c.Both g-C3N4 and Ag3PO4/g-C3N4 exhibited H3-type hysteresis loops in their N2 adsorption and desorption isotherms, characteristic of type IV isotherms.These observations were attributed to the stacking of Ag3PO4 particles and massive g-C3N4.This phenomenon can be attributed to the formation of slit holes in the Ag3PO4 particles and massive g-C3N4.The composite of g-C3N4 and Ag3PO4 exhibited a significantly increased specific surface area.An increased specific surface area enhances the availability of charge transfer channels and catalytic active sites.
The morphology of Ag3PO4 particles, illustrated in Figure 2a, appears irregularly spherical and agglomerated.As shown in Figure 2b, the pure g-C3N4 is an aggregated flocculent structure, which has the typical characteristics of g-C3N4 synthesized by thermal polymerization.When Ag3PO4 and g-C3N4 were combined (Figure 2c), their basic morphological features remain unchanged, resulting in a compact composite structure.In addition, Figure 2d reveals the close contact between Ag3PO4 and g-C3N4, forming a heterojunction with a crystal plane spacing of 0.2689 nm, which is consistent with the (210) crystal plane spacing of Ag3PO4.Notably, the high specific surface area of g-C3N4 facilitates enhanced adsorption of pollutant molecules during photocatalytic reactions.Simultaneously, the heterostructure formed between g-C3N4 and Ag3PO4 facilitates the separation of photogenerated electrons and holes, thereby enhancing the photocatalytic activity [29,30].In summary, the successful construction of the photocatalytic composite Ag3PO4/g-C3N4 heterojunction is demonstrated.

XPS Analysis
The chemical composition and valence states of the various substances were determined using XPS measurements.Figure 3a presents the survey XPS spectrum of the Ag 3 PO 4 /g-C 3 N 4 composite, revealing the presence of oxygen, silver, phosphorus, carbon and nitrogen.Figure 3b shows that the Ag 3d peaks with binding energies at 367.68 and 373.88 eV correspond to the Ag 3d 5/2 and Ag 3d 3/2 characteristic peaks, respectively, indicating the presence of silver in the composite photocatalysts [31,32].As seen in Figure 3c, the peak of the P 2p spectrum at 132.95 eV can be attributed to the phosphorus in Ag 3 PO 4 .The N 1s XPS spectrum (Figure 3d) is decomposed into four peaks.The peak at 397.97 eV corresponds to the sp 2 hybridized nitrogen (C-N=C) in the triazine ring due to the presence of different N-bonds in the repeating unit of the ring.The peaks at 398.85 and 400.57eV are attributed to tertiary nitrogen N-(C) 3 and terminal amino C-N-H, respectively [33,34].The peak at the center of 404.62 eV is attributed to positive charge localization or charging effects in the cyano and heterocyclic rings or N-oxides.The C 1s spectra of Ag 3 PO 4 /g-C 3 N 4 show two peaks (Figure 3e): 287.17 eV, corresponding to sp 2 C atoms bonded to N (C-(N) 3 ), and 285.69 eV, corresponding to amorphous carbon.Figure 3f displays the O 1s spectra.The peaks at 530.61 and 532.49eV are attributed to the oxygen in the Ag 3 PO 4 lattice and the surface oxygen species of the composites, respectively, including hydroxyl or carboxyl oxygen groups [35].It is worth noting that after the combination of two single catalysts, the element binding energy of the composite material shifts compared to that of the pure catalyst, indicating a changed chemical environment and a strong interaction between Ag 3 PO 4 and g-C 3 N 4 .Therefore, the composite material is not a simple physical mixture: Ag 3 PO 4 /g-C 3 N 4 have achieved Z-scheme heterojunction.

XPS Analysis
The chemical composition and valence states of the various substances were determined using XPS measurements.Figure 3a presents the survey XPS spectrum of the Ag3PO4/g-C3N4 composite, revealing the presence of oxygen, silver, phosphorus, carbon and nitrogen.Figure 3b shows that the Ag 3d peaks with binding energies at 367.68 and 373.88 eV correspond to the Ag 3d5/2 and Ag 3d3/2 characteristic peaks, respectively, indicating the presence of silver in the composite photocatalysts [31,32].As seen in Figure 3c, the peak of the P 2p spectrum at 132.95 eV can be attributed to the phosphorus in Ag3PO4.The N 1s XPS spectrum (Figure 3d) is decomposed into four peaks.The peak at 397.97 eV corresponds to the sp 2 hybridized nitrogen (C-N=C) in the triazine ring due to the presence of different N-bonds in the repeating unit of the ring.The peaks at 398.85 and 400.57eV are attributed to tertiary nitrogen N-(C)3 and terminal amino C-N-H, respectively [33,34].The peak at the center of 404.62 eV is attributed to positive charge localization or charging effects in the cyano and heterocyclic rings or N-oxides.The C 1s spectra of Ag3PO4/g-C3N4 show two peaks (Figure 3e): 287.17 eV, corresponding to sp 2 C atoms bonded to N (C-(N)3), and 285.69 eV, corresponding to amorphous carbon.Figure 3f displays the O 1s spectra.The peaks at 530.61 and 532.49eV are attributed to the oxygen in the Ag3PO4 lattice and the surface oxygen species of the composites, respectively, including hydroxyl or carboxyl oxygen groups [35].It is worth noting that after the combination of two single catalysts, the element binding energy of the composite material shifts compared to that of the pure catalyst, indicating a changed chemical environment and a strong interaction between Ag3PO4 and g-C3N4.Therefore, the composite material is not a simple physical mixture: Ag3PO4/g-C3N4 have achieved Z-scheme heterojunction.

Analysis of the Energy Band Structure
The light response characteristics of the samples were analyzed using UV-vis DRS. Figure 4a illustrates that the absorption intensity of the Ag3PO4/g-C3N4 composites changes in the visible range, exhibiting a broader range compared to pure g-C3N4.Furthermore, the addition of Ag3PO4 promoted the absorption of g-C3N4 in the visible range

Analysis of the Energy Band Structure
The light response characteristics of the samples were analyzed using UV-vis DRS. Figure 4a illustrates that the absorption intensity of the Ag 3 PO 4 /g-C 3 N 4 composites changes in the visible range, exhibiting a broader range compared to pure g-C 3 N 4 .Furthermore, the addition of Ag 3 PO 4 promoted the absorption of g-C 3 N 4 in the visible range and improved its utilization, as evidenced by the red-shifted absorption bands observed in the composites [36,37].Moreover, the forbidden band gaps of g-C 3 N 4 , Ag 3 PO 4 and Ag 3 PO 4 /g-C 3 N 4 are 2.68, 2.37 and 2.46 eV, respectively, as determined from the plot of (αhν) 2 versus hν in Figure 4b.It is interesting to note that the forbidden bandwidth of the composites is narrower than that of g-C 3 N 4 , which suggests that the introduction of Ag 3 PO 4 enhances the responsiveness of the photocatalysts to light and facilitates the absorption of lower energy photons.
Molecules 2024, 29, x FOR PEER REVIEW 5 of 13 and improved its utilization, as evidenced by the red-shifted absorption bands observed in the composites [36,37].Moreover, the forbidden band gaps of g-C3N4, Ag3PO4 and Ag3PO4/g-C3N4 are 2.68, 2.37 and 2.46 eV, respectively, as determined from the plot of (αhν) 2 versus hν in Figure 4b.It is interesting to note that the forbidden bandwidth of the composites is narrower than that of g-C3N4, which suggests that the introduction of Ag3PO4 enhances the responsiveness of the photocatalysts to light and facilitates the absorption of lower energy photons.Based on the VB-XPS plots of g-C3N4 and Ag3PO4 (Figure 4c,d), their VB-XPS potentials of g-C3N4 and Ag3PO4 are 0.92 and 3.02 eV, respectively.Therefore, using the formula: where ENHE is the standard hydrogen electrode potential, φ is the electron work function of the XPS analyzer with a value of 4.55, and EVB-XPS is the VB value tested by VB-XPS, the VB values of g-C3N4 and Ag3PO4 were calculated as 1.03 and 3.13 eV, respectively [38,39].Subsequently, the conduction band (CB) values for g-C3N4 and Ag3PO4 were determined to be −1.65 and 0.75 eV, respectively, using the Nernst equation.

Optoelectronic Characteristics
Various techniques including electrochemical impedance spectroscopy, photocurrent testing, and photoluminescence are valuable for analyzing the efficiency of charge separation and transfer.Figure 5a illustrates that the charge transfer impedance of g-C3N4 is larger than that of Ag3PO4.This result is consistent with its weaker transient photocurrent results.Conversely, the impedance arc of the g-C3N4 composite with Ag3PO4 is significantly reduced, indicating lower charge transport resistance in the composite photocata- Based on the VB-XPS plots of g-C 3 N 4 and Ag 3 PO 4 (Figure 4c,d), their VB-XPS potentials of g-C 3 N 4 and Ag 3 PO 4 are 0.92 and 3.02 eV, respectively.Therefore, using the formula: where E NHE is the standard hydrogen electrode potential, φ is the electron work function of the XPS analyzer with a value of 4.55, and E VB-XPS is the VB value tested by VB-XPS, the VB values of g-C 3 N 4 and Ag 3 PO 4 were calculated as 1.03 and 3.13 eV, respectively [38,39].Subsequently, the conduction band (CB) values for g-C 3 N 4 and Ag 3 PO 4 were determined to be −1.65 and 0.75 eV, respectively, using the Nernst equation.

Optoelectronic Characteristics
Various techniques including electrochemical impedance spectroscopy, photocurrent testing, and photoluminescence are valuable for analyzing the efficiency of charge separation and transfer.Figure 5a illustrates that the charge transfer impedance of g-C 3 N 4 is larger than that of Ag 3 PO 4 .This result is consistent with its weaker transient photocurrent results.Conversely, the impedance arc of the g-C 3 N 4 composite with Ag 3 PO 4 is significantly reduced, indicating lower charge transport resistance in the composite photocatalytic material.These findings suggest that the coupling of g-C 3 N 4 with Ag 3 PO 4 can effectively regulate the separation and migration efficiency of the carriers [40,41].
Molecules 2024, 29, x FOR PEER REVIEW 6 of 13 lytic material.These findings suggest that the coupling of g-C3N4 with Ag3PO4 can effectively regulate the separation and migration efficiency of the carriers [40,41].As depicted in Figure 5b, the individual photocurrent of g-C3N4 and Ag3PO4 is low under visible light irradiation conditions.However, when g-C3N4 and Ag3PO4 were composited, the composite material exhibited a significantly increased photocurrent density of 7.5 µA.This suggests that the successful construction of a heterojunction with g-C3N4 and Ag3PO4 effectively enhances the efficiency of photogenerated carrier migration [42,43].This study investigated the separation efficiency of electron hole pairs in photocatalysts using the PL technique.
The PL spectra of g-C3N4, Ag3PO4, and Ag3PO4/g-C3N4 samples are presented in Fig- ure 5c.Pure g-C3N4 exhibits a strong emission peak at approximately 425 nm, resulting from the complex reaction between electron and hole pairs.In contrast, the Ag3PO4/g-C3N4 sample exhibits the weakest PL signal, indicating significant suppression of the recombination of photoexcited carriers [44,45].These findings suggest that the Ag3PO4/g-C3N4 composites are superior to g-C3N4 and Ag3PO4 samples in separating electron hole pairs, consequently enhancing the photocatalytic activity.

Evaluation of Photocatalytic Activity and Stability
The photocatalytic activity of the prepared samples was evaluated by degrading RhB solution under visible light irradiation.From Figure 6a, it can be seen that RhB self-degraded about 3% after 1 h of irradiation under visible light, indicating that the self-degradation of RhB is minimal.This indicates that RhB can be used as a target pollutant for evaluating photocatalytic degradation and catalytic activity.In the absence of light, all samples showed a certain degree of degradation, which might be caused by surface adsorption and photosensitization.While individual samples degraded a small amount of RhB solution, the Ag3PO4/g-C3N4 composite showed superior degradation performance.This enhancement could be attributed to the random stacking of nanosheets after composite formation, which increased the specific surface area.
It can also be seen from Figure 6a that the degradation rate of RhB by g-C3N4 changed minimally after 1 h of light exposure, achieving only 38%; in contrast, Ag3PO4 achieved 85%, and Ag3PO4/g-C3N4 notably reached 99.8%.This indicates that the visible light degradation efficiency of the composite photocatalytic material is significantly improved, which is mainly due to the narrower forbidden band width of the composite photocatalyst.It can better absorb and utilize the visible light, and its optical absorption characteristics are significantly enhanced.The heterojunction facilitates the separation and transmission of photogenerated carriers, enabling the composite material to effectively degrade organic pollutants.This leads to the highest photocatalytic degradation efficiency [46,47].
To further investigate the degradation performance of the composite photocatalysts, As depicted in Figure 5b, the individual photocurrent of g-C 3 N 4 and Ag 3 PO 4 is low under visible light irradiation conditions.However, when g-C 3 N 4 and Ag 3 PO 4 were composited, the composite material exhibited a significantly increased photocurrent density of 7.5 µA.This suggests that the successful construction of a heterojunction with g-C 3 N 4 and Ag 3 PO 4 effectively enhances the efficiency of photogenerated carrier migration [42,43].This study investigated the separation efficiency of electron hole pairs in photocatalysts using the PL technique.
The PL spectra of g-C 3 N 4 , Ag 3 PO 4 , and Ag 3 PO 4 /g-C 3 N 4 samples are presented in Figure 5c.Pure g-C 3 N 4 exhibits a strong emission peak at approximately 425 nm, resulting from the complex reaction between electron and hole pairs.In contrast, the Ag 3 PO 4 /g-C 3 N 4 sample exhibits the weakest PL signal, indicating significant suppression of the recombination of photoexcited carriers [44,45].These findings suggest that the Ag 3 PO 4 /g-C 3 N 4 composites are superior to g-C 3 N 4 and Ag 3 PO 4 samples in separating electron hole pairs, consequently enhancing the photocatalytic activity.

Evaluation of Photocatalytic Activity and Stability
The photocatalytic activity of the prepared samples was evaluated by degrading RhB solution under visible light irradiation.From Figure 6a, it can be seen that RhB self-degraded about 3% after 1 h of irradiation under visible light, indicating that the selfdegradation of RhB is minimal.This indicates that RhB can be used as a target pollutant for evaluating photocatalytic degradation and catalytic activity.In the absence of light, all samples showed a certain degree of degradation, which might be caused by surface adsorption and photosensitization.While individual samples degraded a small amount of RhB solution, the Ag 3 PO 4 /g-C 3 N 4 composite showed superior degradation performance.This enhancement could be attributed to the random stacking of nanosheets after composite formation, which increased the specific surface area.
It can also be seen from Figure 6a that the degradation rate of RhB by g-C 3 N 4 changed minimally after 1 h of light exposure, achieving only 38%; in contrast, Ag 3 PO 4 achieved 85%, and Ag 3 PO 4 /g-C 3 N 4 notably reached 99.8%.This indicates that the visible light degradation efficiency of the composite photocatalytic material is significantly improved, which is mainly due to the narrower forbidden band width of the composite photocatalyst.It can better absorb and utilize the visible light, and its optical absorption characteristics are significantly enhanced.The heterojunction facilitates the separation and transmission of photogenerated carriers, enabling the composite material to effectively degrade organic pollutants.This leads to the highest photocatalytic degradation efficiency [46,47].posite nanofiber material.This suggests its potential as a novel composite photocatalytic material for industrial wastewater purification.The XRD spectra of the composite photocatalysts before and after five degradation cycles are shown in Figure 6f.The intensity of the diffraction peaks of the samples did not decrease significantly, indicating that the crystal structure of the composite photocatalysts remained stable during the catalytic degradation process.To further investigate the degradation performance of the composite photocatalysts, we investigated the kinetics of RhB photocatalytic degradation in the samples.The catalytic degradation process exhibited a one-stage reaction, as indicated by the −ln(C t /C 0 ) versus time plot (Figure 6b).The photocatalytic apparent rate constants of the samples g-C 3 N 4 , Ag 3 PO 4 and Ag 3 PO 4 /g-C 3 N 4 were obtained as 0.00747, 0.02849 and 0.11536 min −1 , respectively.Notably, the degradation rate constants of Ag 3 PO 4 /g-C 3 N 4 were 15.48 and 4.06 times higher than those of g-C 3 N 4 and Ag 3 PO 4 , respectively.
The photocatalytic stability of the samples was evaluated by RhB photocatalytic degradation cycle experiments.Figure 6c displays the degradation efficiencies of Ag 3 PO 4 /g-C 3 N 4 for various concentrations of RhB.The photocatalytic efficiency of the composites decreased with the increase in RhB concentration.In the RhB mass concentration of 16 mg•L −1 system, the degradation efficiency of RhB was maintained at about 95% within 60 min.In the RhB mass concentration of 12, 8 and 4 mg•L −1 system, an almost 100% degradation rate can be achieved within 60 min.Therefore, a high initial concentration of RhB is not conducive to the photocatalytic degradation process.This effect may stem from the high concentration of substrate in the reaction system, which diminishes light absorption and utilization by the material, thereby reducing the excitation energy of photogenerated electron hole pairs and hindering the photocatalytic performance of the system.
As shown in Figure 6d,e, the degradation rate of RhB by Ag 3 PO 4 /g-C 3 N 4 remained above 89.1% after five cycles, demonstrating the high stability and reusability of the composite nanofiber material.This suggests its potential as a novel composite photocatalytic material for industrial wastewater purification.The XRD spectra of the composite photocatalysts before and after five degradation cycles are shown in Figure 6f.The intensity of the diffraction peaks of the samples did not decrease significantly, indicating that the crystal structure of the composite photocatalysts remained stable during the catalytic degradation process.

Photocatalytic Enhancement Mechanism
The mechanisms of the primary active species involved in the photo-catalytic reaction of Ag 3 PO 4 /g-C 3 N 4 were investigated through capture experiments.Figure 7a

Photocatalytic Enhancement Mechanism
The mechanisms of the primary active species involved in the photo-catalytic reaction of Ag3PO4/g-C3N4 were investigated through capture experiments.Figure 7a displays these results.The addition of BQ or IPA led to a decrease in the degradation rate of RhB from 99.8% to 35.1% and 23.9%, respectively.This indicates that •OH and − Ο ⋅ 2 play cru- cial roles in the degradation of RhB.This result is plausible due to the ECB value of −1.65 eV for g-C3N4, which facilitates the highly reductive nature of photogenerated electrons capable of reducing O2 to − Ο ⋅ 2 (O2/ − Ο ⋅ 2 (−0.33 eV vs. NHE)).Moreover, Ag3PO4 has an EVB value of 3.13 eV (H2O/•OH (2.77 eV vs. NHE)) and can oxidize H2O to •OH.Adding EDTA-2Na did not reduce the breakdown of RhB much, indicating that h + is not the main factor in this process.In order to investigate the possible photocatalytic mechanism of the Ag3PO4/g-C3N4 heterojunction, the flat band potential of Ag3PO4/g-C3N4 was characterized.Mott-Schottky (M-S) curves of g-C3N4 and Ag3PO4 were examined at a frequency of 1000 Hz to determine their semiconductor properties.As shown in Figure 7b, the M-S curve of g-C3N4 exhibits a positive slope, indicating it is an n-type semiconductor.The M-S curve of Ag3PO4 showed a negative slope, confirming its p-type semiconductor nature.This suggests the formation of a p-n heterojunction between g-C3N4 and Ag3PO4.In addition, the flat-band potentials (Efb,Ag/AgCl) of g-C3N4 and Ag3PO4 measured at pH = 7, Ag/AgCl electrode conditions were −1.45 and 0.55 eV, respectively.These Efb values were subsequently converted to the standard hydrogen electrode potential by the equation Efb,NHE = Efb,Ag/AgCl ± 0.197, from which the Efb,NHE was calculated to be −1.65 and 0.75 eV for g-C3N4 and Ag3PO4, respectively [48,49].Therefore, based on the above valence band values of g-C3N4-VB and Ag3PO4-VB (1.03 and 3.13 eV, respectively), the band gap of g-C3N4 and Ag3PO4 can be calculated by the formula: EVB − ECB = Eg.The band gaps of g-C3N4 and Ag3PO4 are 2.68 and 2.38 eV, respectively, which is in agreement with Figure 4b.
Determining the work function difference between semiconductor materials is crucial In order to investigate the possible photocatalytic mechanism of the Ag 3 PO 4 /g-C 3 N 4 heterojunction, the flat band potential of Ag 3 PO 4 /g-C 3 N 4 was characterized.Mott-Schottky (M-S) curves of g-C 3 N 4 and Ag 3 PO 4 were examined at a frequency of 1000 Hz to determine their semiconductor properties.As shown in Figure 7b, the M-S curve of g-C 3 N 4 exhibits a positive slope, indicating it is an n-type semiconductor.The M-S curve of Ag 3 PO 4 showed a negative slope, confirming its p-type semiconductor nature.This suggests the formation of a p-n heterojunction between g-C 3 N 4 and Ag 3 PO 4 .In addition, the flat-band potentials (E fb,Ag/AgCl ) of g-C 3 N 4 and Ag 3 PO 4 measured at pH = 7, Ag/AgCl electrode conditions were −1.45 and 0.55 eV, respectively.These E fb values were subsequently converted to the standard hydrogen electrode potential by the equation E fb,NHE = E fb,Ag/AgCl ± 0.197, from which the E fb,NHE was calculated to be −1.65 and 0.75 eV for g-C 3 N 4 and Ag 3 PO 4 , respectively [48,49].Therefore, based on the above valence band values of g-C 3 N 4 -VB and Ag 3 PO 4 -VB (1.03 and 3.13 eV, respectively), the band gap of g-C 3 N 4 and Ag 3 PO 4 can be calculated by the formula: E VB − E CB = E g .The band gaps of g-C 3 N 4 and Ag 3 PO 4 are 2.68 and 2.38 eV, respectively, which is in agreement with Figure 4b.
Determining the work function difference between semiconductor materials is crucial for analyzing the direction of charge transfer at the material interface.Figure 7c,d depicts the work functions of g-C 3 N 4 and Ag 3 PO 4 obtained through small-angle valence band XPS spectroscopy (VB-XPS) measurements.When two semiconductor materials come into contact, electrons tend to migrate toward the material with the higher work function.Conversely, the material with the smaller work function tends to lose electrons until the Fermi energy levels of the two materials reach equilibrium.Consequently, the surface of the material with the lower work function becomes positively charged, while the surface of the material with the higher work function becomes negatively charged, creating a built-in electric field at the contact interface.According to the equation Φ = ∆V + φ (Φ is the work function of the sample, φ is 4.55 eV), ∆V is obtained from the distance between IP1 and IP2 (IP1 is the point of change of the binding energy with respect to the baseline and IP2 is the midpoint of the corresponding Fermi fringe curve) [50].The distances between the two IP points of g-C 3 N 4 and Ag 3 PO 4 are calculated as 1.24 and 2.16 eV, respectively.Therefore, the work functions of g-C 3 N 4 and Ag 3 PO 4 are calculated as 5.79 and 6.71 eV, respectively.
Theoretically, due to g-C 3 N 4 's higher energy band edge and Fermi energy level compared to Ag 3 PO 4 , there is a high probability for the formation of Z-scheme complexes between these two materials.When the two semiconductors are in contact, the band edges in g-C 3 N 4 continuously bend upward toward the interface, while the band edges in Ag 3 PO 4 bend downward to the interface.As a result of the disruption of the charge transfer channel at the interface, photogenerated electrons from Ag 3 PO 4 and holes from g-C 3 N 4 may form complexes with each other instead of migrating into the energy band of the opposite semiconductor.Therefore, the electrons of g-C 3 N 4 with high redox capacity and the holes of Ag 3 PO 4 can be retained for the photocatalytic process, following the typical Z-scheme mechanism (Figure 8) [51][52][53].
Molecules 2024, 29, x FOR PEER REVIEW 9 of 13 for analyzing the direction of charge transfer at the material interface.Figure 7c,d depicts the work functions of g-C3N4 and Ag3PO4 obtained through small-angle valence band XPS spectroscopy (VB-XPS) measurements.When two semiconductor materials come into contact, electrons tend to migrate toward the material with the higher work function.Conversely, the material with the smaller work function tends to lose electrons until the Fermi energy levels of the two materials reach equilibrium.Consequently, the surface of the material with the lower work function becomes positively charged, while the surface of the material with the higher work function becomes negatively charged, creating a built-in electric field at the contact interface.According to the equation Φ = ΔV + φ (Φ is the work function of the sample, φ is 4.55 eV), ΔV is obtained from the distance between IP1 and IP2 (IP1 is the point of change of the binding energy with respect to the baseline and IP2 is the midpoint of the corresponding Fermi fringe curve) [50].The distances between the two IP points of g-C3N4 and Ag3PO4 are calculated as 1.24 and 2.16 eV, respectively.Therefore, the work functions of g-C3N4 and Ag3PO4 are calculated as 5.79 and 6.71 eV, respectively.
Theoretically, due to g-C3N4's higher energy band edge and Fermi energy level compared to Ag3PO4, there is a high probability for the formation of Z-scheme complexes between these two materials.When the two semiconductors are in contact, the band edges in g-C3N4 continuously bend upward toward the interface, while the band edges in Ag3PO4 bend downward to the interface.As a result of the disruption of the charge transfer channel at the interface, photogenerated electrons from Ag3PO4 and holes from g-C3N4 may form complexes with each other instead of migrating into the energy band of the opposite semiconductor.Therefore, the electrons of g-C3N4 with high redox capacity and the holes of Ag3PO4 can be retained for the photocatalytic process, following the typical Z-scheme mechanism (Figure 8) [51][52][53].

Preparation of Samples
Typically, melamine (15.0462 g) was ground into a porcelain crucible, covered with a lid and heated to 550 • C for 4 h at a rate of 5 • C/min.After it was cooled to room temperature, the yellow-colored lumps of g-C 3 N 4 were obtained, and the sample of g-C 3 N 4 was obtained by grinding it into powder.
The g-C 3 N 4 powder (0.1998 g) and AgNO 3 (1.0010g) were dissolved in 100 mL of deionized water.An amount of aqueous disodium hydrogen phosphate dodecahydrate (1.7899 g) and polyvinylpyrrolidone (1.2002 g) were added dropwise to the above solution under vigorous stirring.Stirring continued for 1 h, 150 mL of the solution was placed in a high-pressure reactor 200 • C constant temperature for 24 h.Then, the mixture was cooled to room temperature, filtered, and the resulting filtrate was washed three times with deionized water and then three times with ethanol.Finally, the filtrate was dried in a vacuum drying oven at 60 • C for 12 h.The sample was prepared as Ag 3 PO 4 /g-C 3 N 4 .The Ag 3 PO 4 sample was synthesized in the same manner, except that g-C 3 N 4 was not included.

Characterization of the Samples
The physical purity and crystal structure of the samples were examined using an X-ray diffractometer (XRD, Rigaku Ultima IV, Japan Co., Ltd., Tokyo, Japan).The morphology of the samples was investigated with a scanning electron microscope (FE-SEM, NANOSEM 450, FEI Co., Ltd., Hillsboro, OR, USA).Additionally, the samples were characterized by TEM using a high-resolution electron microscope (HRTEM, H-7000FA, Hitachi, Tokyo, Japan).A UV-visible spectrophotometer (UV-Vis DRS, UV-2550, Shimadzu, Japan, Kyoto) was used to characterize the energy band structure.Photoluminescence spectra were measured using a fluorescence spectrophotometer (PL, Jasco FP-6500, Tokyo, Japan).X-ray photoelectron spectroscopy (XPS, Axis Ultra) was used to collect data on the chemical composition and elemental valence of the sample surface.The samples' photocurrent (PC) and electrochemical impedance (EIS) were measured using an electrochemical workstation (CHI660B).A photocatalytic device (ZQ-GHX-V, Zhenqiao, Shanghai, China) was used to test the photocatalytic performance.

Evaluation of Photocatalytic Performance and Stability
The photocatalytic activity of the samples was investigated through the application of a photocatalytic degradation process to rhodamine B. The samples (100 mg each) were dispersed in 50 mL of rhodamine B solution at a concentration of 4 mg•L −1 .The adsorption equilibrium was reached after 30 min of dark adsorption.Photocatalytic degradation experiments were conducted at room temperature utilizing a 500 W xenon lamp with a 420 nm filter as the light source.
Samples were collected every 10 min.The supernatant was then centrifuged, filtered, and its concentration was determined using spectrophotometry.The stability of the photocatalysts' degradation cycle was investigated through cyclic photocatalytic experiments.The photocatalytic activity was examined for five consecutive degradation experiments under the same conditions.

Cavity and Radical Trapping Experiments
Major active species during photocatalytic reactions were identified by hole and radical trapping experiments.Isopropanol (IPA) was used as the hydroxyl radical (•OH) trapping agent; p-benzoquinone (BQ) was used as the superoxide anion radical (•O − 2 ) trapping agent; and disodium ethylenediaminetetraacetate (EDTA-2Na) was used as the hole (h + ) trapping agent.The relevant trapping agent (1 µmol•L −1 ) was added during the photocatalytic degradation reaction of RhB under the same conditions as those used for the evaluation of photocatalytic activity to obtain curves of RhB concentration versus time.The main active species of the photocatalytic reaction was determined by comparing the activity changes before and after the addition of the trapping agent.

Conclusions
In conclusion, an Ag 3 PO 4 /g-C 3 N 4 heterojunction photocatalyst was prepared via a simple calcination and hydrothermal synthesis method to efficiently degrade RhB.The heterojunction between g-C 3 N 4 and Ag 3 PO 4 significantly enhances the separation efficiency and transport speed of the photogenerated carriers.The synthesized Ag 3 PO 4 /g-C 3 N 4 composite photocatalysts exhibited significantly enhanced activity in RhB decomposition under visible light irradiation.Capture experiments indicated that the degradation process was primarily influenced by •O − 2 and •OH, followed by H + .Finally, a possible Z-scheme degradation mechanism of organic pollutants by the Ag 3 PO 4 /gC 3 N 4 composite photocatalyst is proposed based on the work function analysis and energy band theory.The composite photocatalyst exhibits high stability and reusability, making it a promising candidate for practical photocatalytic applications in industrial wastewater purification.

Figure 1 .
Figure 1.(a) XRD patterns of the different samples.(b) EDS pattern of the Ag 3 PO 4 /g-C 3 N 4 .(c) N 2 adsorption and desorption isotherms of g-C 3 N 4 and Ag 3 PO 4 /g-C 3 N 4 .

Figure 6 .
Figure 6.(a) Degradation curves of RhB by samples under visible light irradiation.(b) Corresponding first-order kinetics of samples.(c) The concentration effects of RhB on the photodegradation efficiency of the composite sample.(d,e) Five cycling runs of the composite sample for RhB degradation.(f) XRD patterns of Ag3PO4/g-C3N4 before and after the cyclic test.

Figure 6 .
Figure 6.(a) Degradation curves of RhB by samples under visible light irradiation.(b) Corresponding first-order kinetics of samples.(c) The concentration effects of RhB on the photodegradation efficiency of the composite sample.(d,e) Five cycling runs of the composite sample for RhB degradation.(f) XRD patterns of Ag 3 PO 4 /g-C 3 N 4 before and after the cyclic test.
displays these results.The addition of BQ or IPA led to a decrease in the degradation rate of RhB from 99.8% to 35.1% and 23.9%, respectively.This indicates that •OH and •O − 2 play crucial roles in the degradation of RhB.This result is plausible due to the E CB value of −1.65 eV for g-C 3 N 4 , which facilitates the highly reductive nature of photogenerated electrons capable of reducing O 2 to •O − 2 (O 2 /•O − 2 (−0.33 eV vs. NHE)).Moreover, Ag 3 PO 4 has an E VB value of 3.13 eV (H 2 O/•OH (2.77 eV vs. NHE)) and can oxidize H 2 O to •OH.Adding EDTA-2Na did not reduce the breakdown of RhB much, indicating that h + is not the main factor in this process.Molecules 2024, 29, x FOR PEER REVIEW 8 of 13

Figure 8 .
Figure 8. Schematic illustration of the degradation mechanism.Figure 8. Schematic illustration of the degradation mechanism.

Figure 8 .
Figure 8. Schematic illustration of the degradation mechanism.Figure 8. Schematic illustration of the degradation mechanism.